1. Field of the Invention
The present invention relates to a thin film magnetic head device, and more particularly to a thin film magnetic head device comprising a reading thin film magnetic head element whose electric equivalent circuit is represented by a series circuit of an equivalent voltage source and a series resistor having a resistance RH and a parallel capacitor having a capacitance C and connected in parallel with said series circuit.
2. Related Art Statements
A typical thin film magnetic head device having a thin film magnetic head element whose electric equivalent circuit is represented by a series circuit of an equivalent voltage source and a series resistor and a parallel capacitor connected in parallel with said series circuit is a magneto-resistive type thin film magnetic head device.
FIG. 1 is a circuit diagram showing an electric equivalent circuit of a thin film magnetic head element of a known magneto-resistive type thin film magnetic head device. The thin film magnetic head element is represented by a series circuit of an equivalent voltage source 1 and a series resistor 2 having a resistance RH and a parallel capacitor 3 having a capacitance C and connected in parallel with said series circuit. Both ends of the parallel capacitor 3 are connected to an externally provided reading circuit. In FIG. 1, the externally arranged reading circuit is represented by an input resistor 4 having a resistance Rin.
The typical magneto-resistive type thin film magnetic head clement whose equivalent circuit is expressed by a series circuit of the voltage source 1 and series resistor 2 and the parallel capacitor 3 is a giant magneto-resistive thin film magnetic head element. Upon compared with a normal anisotropic type magneto-resistive thin film magnetic head element, the magneto-resistive thin film magnetic head element has an extremely high resistance change ratio by a magnetic field and can be applied to a high recording density hard disk device. There has been realized a very high surface recording density up to 15 Gb/in2.
The giant magneto-resistive thin film magnetic head element has been realized as anti-ferromagnetic coupling type, induction ferromagnetic type, spin-valve type and glanular type. By using such giant magneto-resistive thin film magnetic head element, it is possible to realize an extremely narrow track width of sub-micron order However, their resistance change ratios do not exceed 10%, and this results in that it is difficult to obtain a sufficiently large reproduced signal if a track is further narrowed. That is to say, the known giant magneto-resistive thin film magnetic head elements could not satisfy severe requirements for extremely high recording density.
There has been proposed a tunneling type giant magneto-resistive element which can produce a large reproduced signal for an extremely narrow track width. Such a tunneling type giant magneto-resistive thin film magnetic head is described in, for instance Japanese Patent Laid-open Publication Kokai Hei 11-135857. FIG. 2 shows a principal structure of such a tunneling type magneto-resistive element. This element includes a first magnetic layer 5 which serves as a free layer whose direction of magnetization is changed in accordance with a magnetic field produced by a magnetic record medium, a second magnetic layer which functions as a pin layer whose direction of magnetization is fixed, an insulating layer 7 arranged between the first and second magnetic layers 5 and 6, and a anti-ferromagnetic layer 8 provided on the second magnetic layer 6. The first and second magnetic layers 5 and 6 have a stacked structure of Co layer and Ni—Fe layer for attaining a large resistance change ratio, and the insulating layer 7 is formed by an Al2O3 layer manufactured by oxidizing an aluminum layer at the room temperature.
The number of electrons tunneling the insulating layer 7 becomes maximum when the direction of magnetization of the first magnetic layer 5 serving as the free layer is identical with that of the second magnetic layer 6 serving as the pin layer under a condition that a voltage is applied perpendicularly to the insulating layer 7. When the direction of magnetization of the first magnetic layer 5 is opposite to that of the second magnetic layer 6, the number of tunneling electrons becomes minimum. In this manner, a resistance value changes from the maximum value to the minimum value in accordance with the direction of magnetization of the first magnetic layer 5. Theoretically, a resistance change ratio amounts to 50%, and in practice, there has been proposed tunneling type giant magneto-resistive thin film magnetic head element having a resistance change ratio not less than 20%, and such tunneling type elements may be applied to an extremely narrow track width.
An output transfer function Av of a thin film magnetic head element whose electric equivalent circuit is represented by a series circuit of the equivalent voltage source 1 and a series resistor 2 and the parallel capacitor 3 connected in parallel with said series circuit as illustrated in FIG. 1 is given by the following equation (1):                               A          V                =                  1                                    1              +                                                (                                      ω                    /                                          ω                      H                                                        )                                2                                                                        (        1        )            Here, E is a voltage of the equivalent power source 1. V is an output voltage of the thin film magnetic head element, ωH is an angular frequency determined by the capacitance C of the parallel capacitor 3 and the resistance RH of the series resistor 2 and is expressed by ωH=1/C RH. It is assumed that the resistance Rin of the input resistance 4 is infinite.
As can be understood from the above equation (1), the output transfer function Av of the tunneling type magneto-resistive thin film magnetic head element is decreased in accordance with an increase in the resistance RH of the series resistor 2. Therefore, the reproduced output signal is decreased in accordance with an increase in the operation frequency, and an operation frequency range is narrowed. In order to increase reproduction speed, it is necessary to extend the operation frequency range up to a high frequency, and it is desired to obtain a large output signal at a high frequency. For instance, in Japanese Patent Laid-open Publication Kokai 2000-113433, there is proposed a thin film magnetic head device having an operation frequency more than 150 MHz.
In order to realize an extremely narrow record track, the thin film magnetic head element has to be miniaturized. The, the resistance RH of the series resistor 2 is liable to be increased. Particularly, in the tunneling type giant magneto-resistive thin film magnetic head element, the series resistor 2 usually has a high resistance RH due to the fact that the current flows perpendicularly to the insulating layer 7, and amounts to several hundreds Ω to several KΩ. Therefore, the operation frequency range is limited largely. In order to attain the high speed operation by increasing the operation frequency, the output signal is liable to be lowered, and inherent merits of the tunneling type magneto-resistive thin film magnetic head element might be lost.